1. Technical Field
The present invention relates to piezoelectric ultrasonic atomizers, particularly of the type having an atomizing surface at a tip of a reduced diameter amplifying probe at one end of a transducer and a coaxial fluid delivery channel extending from the other end of the transducer to the atomizing surface.
2. Background Art
Piezoelectric ultrasonic atomizers are finding increasing use in industrial applications where liquid materials must be delivered in the form of a very fine spray or mist. The design and construction of such atomizers is described in U.S. Pat. No. 4,337,896 of Berger et al. A typical arrangement is to sandwich a flat electrode between two disks of piezoelectric material, such as lead zirconate titanate, to form a driving element, and then to clamp the driving element between a cylindrical front amplifying horn and a cylindrical rear dummy section. The amplifying horn is provided with a reduced diameter probe having an atomizing surface at its tip. The amplification of vibrational amplitude obtained at the atomizing surface is approximately equal to the ratio between the respective cross-sectional areas of the cylindrical portion of the front horn and of the end of the probe.
In the type of atomizer shown in U.S. Pat. No. 4,337,896, the necessary clamping pressure on the driving element is obtained by providing circumferential flanges on the adjacent ends of the front and rear sections and drawing the flanges together with a circle of bolts. The flanges also provide an annular bearing area for compressing an elastomeric gasket ring, to prevent liquid spray from contacting the outer peripheries of the piezoelectric disks. The sealing effectiveness of such a gasket is an important factor in extending the operating life of the atomizer.
The clamping flange design has drawbacks, however. To reduce internal losses, the front and rear horns should each be made as a single piece. It is wasteful to have to start with stock having an outer diameter equal to the flange diameter and then machine as much as two-thirds of it away. More importantly, the size of droplets formed by an ultrasonic atomizer varies inversely with the frequency of the unit. To obtain very small particles in the micron range, it is necessary to use very high frequencies, well over 100 kHz. To avoid significant transverse wave motion in the transducer, however, the transverse dimensions of the front and rear sections should be less than one-quarter wavelength.
As an example, in titanium a quarter wavelength at frequencies about 100 kHz is less than one centimeter. It is desirable to have the ratio between cylindrical section diameter and probe tip diameter be as large as possible, for increased amplification. At the same time, the atomizing surface should be large enough to handle a reasonable flow and the probe must be sturdy enough to resist breaking in operation. These factors make it undesirable to use up part of the diametral dimensions for clamping flanges.
An alternative arrangement for clamping a cylindrical atomizing transducer and concurrently protecting the piezoelectric elements from liquid contamination is disclosed in U.S. Pat. No. 3,861,852 of Berger. In this arrangement, a cylindrical transducer is inserted into a cup, and the transducer elements are clamped together by force exerted upon a flange on the rear dummy section by a cap threaded into the cup, with the front face of the transducer bearing against the base of the cup. O-rings at the clamping surfaces seal the transducer inside the cup from liquid spray delivered from the tip of a probe extending through an opening in the base of the cup. It is difficult to apply and maintain the proper clamping pressure on the piezoelectric driving element with this arrangement, however, and the end clamping can introduce significant damping and thereby reduce efficiency of the transducer.
Although liquid is fed to the above-described atomizers through a radial passage that intersects an axial channel in the front horn of the transducer, it is also known, for example from U.S. Pat. No. 4,352,459 of Berger et al., to feed the liquid axially through the rear section of a flange-clamped transducer. It is necessary in this design, however, to provide an annular sealing gasket between the feed tube and the inner circumferences of the piezoelectric disks, thus reducing the potential cross sectional area of the disks and thereby the available vibrational driving power. It is also known to clamp the driving element of a piezoelectric transducer by means of a solid central bolt, as in U.S. Pat. Nos. 3,368,085 of McMaster et al.; 3,396,285 of Minchenko, 3,689,783 of Williams, and 3,694,675 of Loveday. The transducers of these patents are not fluid feed atomizers, however.